In regard to the method of measuring information on diffusion of particle groups, the inventors of the present invention have proposed an apparatus and a method for evaluating the information on the diffusion of particles in a sample. That is, a container retains a sample in which particle groups are dispersed in a medium. A comb-type electrode pair has plural electrode pieces which are electrically connected at one ends thereof. The electrode pair is disposed in the container such that the other ends of electrode pieces of each electrode oppose each other with a minute gap. Applying voltage to the electrode pair generates electric field distribution regularly lined between the opposed electrode pieces to provide the particle groups in the sample inside the container with a dielectrophoretic force. Density distribution of the particle groups caused by the dielectrophoretic force generates a diffraction grating. After the diffraction grating is generated, the application of voltage to the electrode pair is stopped to diffuse the particle groups and then the diffraction grating is disappeared. Irradiating a beam of light to an area where the diffraction grating is generated in the container and detecting intensity of the obtained diffracted light between the generation and the disappearance of the diffraction grating. And information on the diffusion of particles in the sample is evaluated from a temporal variation of intensity of the diffracted light in disappearing process of the diffraction grating.
Further, the inventors widen width (length) of a diffraction grating generated by density distribution of particle groups in order that more components of the diffracted light may be contained in detected light without narrowing irradiation light to enhance sensitivity in measurements. And they propose also an electrode pattern for applying voltage to induce dielectrophoresis of particles by which diffracted light by diffraction grating generated by particle density can be measured separately from diffracted light by the electrode pattern (for example, refer to ‘the examination of nano-particle measuring method using a transient diffraction grating’ by Yukihisa Wada et al in the Proceedings of the 52nd spring meeting of the Japan Society of Applied Physics, separate Vol. 3 29 Mar. 2005, page 1142,31p-ZF-21. of the 52nd OYO BUTURIGAKU KANKEI RENGO).
In concrete, a sample to be measured formed by dispersing particle groups in a liquid or gel is retained in a container 1 as shown in a vertical section of FIG. 6 and a predetermined voltage is applied to an electrode pair 2 provided inside the container 1 to induce dielectrophoresis of the particle groups and thereby generate a diffraction grating by a density distribution thereof. A pattern illustrated in FIG. 7 is used as the electrode pair 2, and it consequently becomes possible to measure a diffracted light by the particle density diffraction grating separately from a diffracted light by the electrode pair 2.
As illustrated in FIG. 7, the electrode pair 2 is composed of electrodes 21 and 22. The electrode 21 is composed of plural parallel electrode pieces 21a and a connection area 21b that electrically connects the electrode pieces 21a. And the electrode 22 is composed of plural parallel electrode pieces 22a and a connection area 22b that electrically connects the electrode pieces 22a. The teeth of comb-electrodes belonging to 21 and 22 side are arranged as 21a-21a-22a-22a-21a-21a-22a-22a-and so on, as in FIG. 7. Two linear electrode pieces 21a or 22a are adjacently arrayed in the electrode area. There is no electrode piece in the non-electrode area. Two electrode pieces 21a or 22a of the electrode area are disposed in the non-electrode area of other electrode. As a whole two electrode pieces 21a and 22a are alternately disposed in parallel with a constant gap.
When an alternate voltage for example is applied between the electrodes 21 and 22, an electric field distribution corresponding to the electrode pattern is formed in the sample inside the container 1 by the electrode pattern. And the particle groups inside the sample are moved by the dielectrophoresis based on the electric filed distribution to generate a density distribution of the particle groups. In the electrode pattern illustrated in FIG. 7, a high-density area P of the particles is formed in an area where reverse polarity electrode pieces are adjacent. And the grating gap of the diffraction grating formed by the high-density area P of the particles consequently becomes twice as wide as the grating gap of the diffraction grating formed by the electrode pieces 21a or 22a. And the grating constants are as a result different between them. Diffracted light from the diffraction grating formed by the density distribution of the particle groups is defined to be diffracted light of a specific order by grating constant of the diffraction grating by the density distribution and grating constant of the diffraction grating by the electrode pieces. The diffracted light of a specific order appears in direction where the diffracted light by the diffraction grating formed by the electrode pieces does not exist.
In the example of FIG. 7, the diffracted light of [2m+1] order (m: integral number) by the diffraction grating formed by the density distribution of the particles appears in the direction where the diffracted light by the diffraction grating formed by the electrode pieces does not exist. If a detection optical system is located in the direction, background light contained in detected light by the detection optical system becomes background light composed of a scattered light and so forth. And background noise is accordingly kept low and it becomes possible to measure the diffracted light from the diffraction grating by the density distribution of the particle groups under a satisfactory background noise.